Space shuttle Discovery's main mission is to install the Kibo lab module on the International Space Station — but what's really captivated the world's attention is a more fundamental issue.

Less than a week before Discovery lifted off from Cape Canaveral Saturday evening, the Russian-built zero-gravity toilet aboard the space station broke down.

The, ahem, solid-waste disposal aspect of the loo is still working fine, but extra water has to be pumped by hand into the system for the liquid-waste part to function.

A new pump was flown from Moscow to Florida via diplomatic pouch for last-minute addition to Discovery's cargo.

Now a Russian space official says that if the repair, scheduled for Wednesday, doesn't fix the situation, the space station's crew of three may have to come home early.

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"It's true, we have a problem with the flushing system. This is a serious matter," Vladimir Solovyov, chief ground control official for the Russian section of the space station, told the Interfax news agency, according to Agence France-Presse. "In such circumstances, there's even the possibility of an emergency departure from the station."

That may be a bit of an exaggeration.

"Although it's true that if the toilet broke completely, that would qualify as a reason to evacuate the space station, that's not the situation right now," explained Clara Moskowitz, a reporter for Space.com who's been covering the current shuttle mission.

"The toilet is currently working — it's simply inconvenient to use," she added. "So even if the new replacement pump doesn't fix the issues, the astronauts wouldn't have to evacuate the station because the toilet is still usable."

Until Discovery docked Monday, the two Russian cosmonauts and NASA's Garrett Reisman were using the toilet aboard the Soyuz escape vehicle that's always parked at the space station, as well as using the "manual mode" to flush the broken one.

With the arrival of Discovery, all bodily functions shifted to the shuttle's own plumbing system.

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The current toilet, even half broken, is still an improvement over waste disposal of previous space eras. The toilet aboard Skylab in 1973-74 was an ancestor of today's john, but it all ended up in a bag instead of a proper waste-disposal system.

Before that, astronauts did their business right into plastic bags — and indeed, Discovery brought up a load of "Apollo bags" this time just in case the toilet repairs fail.

Whatever the outcome of the orbital plumbing, ISS crewmembers have a brighter, cleaner future to look forward to.

"The reason a broken toilet could potentially cause an evacuation is because the space station only has one," said Moskowitz.

That's about to change. Later this year, another shuttle mission will bring up and install a newer, more efficient second toilet, again built by the Russians, this time at a cost of $19 million.

And that will effectively transform the space station from a one- to a two-bathroom apartment, albeit a phenomenally expensive one with an unbeatable view.